Reliably Learning the ReLU in Polynomial Time.

We give the first dimension-efficient algorithms for learning Rectified Linear Units (ReLUs), which are functions of the form $mathbf{x} mapsto max(0, mathbf{w} cdot mathbf{x})$ with $mathbf{w} mathbb{S}^{n-1}$. algorithm works in the challenging Reliable Agnostic learning model of Kalai, Kanade, and Mansour (2009) where the learner is given access to a distribution $cal{D}$ on labeled examples but the labeling may be arbitrary. We construct a hypothesis that simultaneously minimizes the false-positive rate and the loss on inputs given positive labels by $cal{D}$, for any convex, bounded, and Lipschitz loss function. The algorithm runs in polynomial-time (in $n$) with respect to any distribution on $mathbb{S}^{n-1}$ (the unit sphere in $n$ dimensions) and for any error parameter $epsilon = Omega(1/log n)$ (this yields a PTAS for a question raised by F. Bach on the complexity of maximizing ReLUs). These results are in contrast to known efficient algorithms for reliably learning linear threshold functions, where $epsilon$ must be $Omega(1)$ and strong assumptions are required on the marginal distribution. We can compose our results to obtain the first set of efficient algorithms for learning constant-depth networks of ReLUs. Our techniques combine kernel methods and polynomial approximations with a dual-loss approach to programming. As a byproduct we obtain a number of applications including the first set of efficient algorithms for convex piecewise-linear fitting and the first efficient algorithms for noisy polynomial reconstruction of low-weight polynomials on the unit sphere.